1. Field of the Invention
The present invention relates to defect repair, particularly in EUVL multilayers, and more specifically, it relates to methods for determining the location of such defects.
2. Description of Related Art
Extreme ultraviolet (EUV) lithography is the top contender for next generation lithography in high-volume semiconductor manufacturing for the 35 nm node and beyond. It utilizes 13.4 nm radiation as the exposure light source and employs Mo—Si multilayer stacks as the reflector for both optic mirrors and mask blanks.
Referring to FIG. 1A, EUV mask blanks are fabricated by depositing a reflective Mo/Si multilayer film 10 onto a super-polished substrate 12. Localized defects in this thin film can significantly alter the reflected field and introduce errors in the printed image. When input beam 14 is reflected, as depicted by reflected beam 18, from the multilayer film 10 of FIG. 1A, a defect 16 located near the top of the multilayer prevents a portion of the beam from being reflected, resulting in a reduction in amplitude of reflected beam 18. When input beam 20 is reflected, as depicted by reflected beam 22, from the multilayer film 20 (on substrate 26) of FIG. 1A, a defect 28 located near the lower portion of the multilayer prevents a portion of beam from being reflected, resulting in a phase defect in reflected beam 22. Several techniques for repairing localized defects have been suggested (see U.S. patent application Ser. No. 09/669,390, titled: “Repair of phase defects in EUVL mask blanks using spatially-resolved, enhanced interdiffusion in the multilayer coatings,” incorporated herein by reference and U.S. patent application Ser. No. 09/896,722, titled: “Method to repair localized amplitude defects in EUVL mask blanks,” incorporated herein by reference. The applicability of these techniques depends on the position of the defects in the multilayer stack. Phase defects 30 and 32, as shown in FIGS. 2A and 2B respectively, are repaired by contracting the volume above the defect through local heating, as shown in FIG. 2B. On the other hand, an amplitude defect 40, as shown in FIG. 3A, is repaired by removing the defect along with the surrounding multilayer altogether to leave a removed area 42, as shown in FIG. 3B, and the top surface of the removed area is capped with a protective layer 44 to prevent oxidation, as shown in FIG. 3C. The right repair technique is selected, depending on the out-of-plane position of the defect in the multilayer, to prevent the repair zone from becoming worse or even un-repairable.
Several techniques and tools have been developed to determine the in-plane position of defects. However, little work has been done to determine the out-of-plane defect position in a non-destructive way. Other techniques, such as cross-sectional transmission electron microscopy (TEM) using a focused ion beam (FIB) are destructive and render the mask useless. An aerial image microscope (AIMS tool) can image the defect, but cannot uniquely determine the position of the defect in the multilayer stack.
It is desirable to provide a cost-efficient, non-destructive way to locate, characterize and repair defects in thin films.